The need sometimes arises for a machine component or other structure, which consists of a base or body of one material, carrying an external layer providing a face that consists of another material. Specifically, for example, the need frequently arises for rollers that are faced with a material that will resist wear for substantial intervals of rotary engagement with abrasive substances. As a specific example of a facing material, carbides, e.g., tungsten carbide materials, have been determined to be exceedingly hard and wear resistant. Consequently, a considerable need exists for machine components of steel, that are faced with such carbides.
Various techniques have been proposed to surface machine components with tungsten carbide material. For example, it has been proposed to apply weld metal (bearing tungsten carbide particles) to provide a layer carrying the hard particles. However, as the tungsten carbide particles are substantially heavier than the weld metal, they tend to sink within the weld puddle at the point of application and accordingly are somewhat removed from the exposed surface. Also abrasives, fluxes and flame-spraying techniques have been proposed for adhering carbides to molds, as disclosed in U.S. Pat. Nos. 3,553,905 (Lemelson) and 3,028,644 (Waldrop). However, such methods tend to inhibit production rates. Thus, difficulties have continued with various systems proposed for accomplishing a roller that is faced with tungsten carbide material. Consequently, a substantial need exists for an economical and effective system for the production of such hard-faced structures.
In general, one aspect of the present invention involves providing particulate-faced rollers by the use of centrifugal casting techniques. More specifically, a mold is provided to define an internal support surface which is coincident with the desired particulate external surface of the roller to be produced. The mold is then rotated to develop a centrifugal force extending toward its internal support surface. Recognizing prior centrifugal casting techniques, as disclosed in U.S. Pat. Nos. 2,645,558 (Burchartz) and 1,699,612 (Doat) wherein protective mold linings have been proposed; U.S. Pat. No. 3,468,997 (Pickels) wherein particulate matter is oriented and U.S. Pat. No. 2,260,593 (Wittlinger et al.) of concern regarding metal distribution, the present system is quite distinct. That is, a phenomena and characteristics of components employed to accomplish the phenomena, distinguish the present system from prior art methods and techniques. For example, the above-referenced patent 2,645,558 discloses the incorporation of cast iron chips from a mold lining into the casting; however, such was a problem to be avoided and was a surface condition, the casting metal freezing, almost on contact with the chips.
In accordance with the present invention, particles of tungsten carbide are placed in the mold along with molten metal, which is lighter (less dense) than the particles and has a lower melting point. Consequently, the solid particles are held at the support surface. However, the molten metal is characterized and forces are developed to move the molten metal into the interstices between the particles. As the metal wets the particles, upon solidification, they are securely held at the face of the casting in a dense matrix of the base metal which is integral with a metal body.
In addition to the problems treated above, some further difficulties are encountered in the formation of rollers subsequent the discovery of the centrifugal casting technique. Specifically, at best it is impractical to machine the tungsten-carbide surface of the roller due to the nature of that substance. Furthermore, the centrifugal casting has a side effect that imparts a characteristic of poor machinability to the matrix metal. That is, the rapid cooling of the casting results in a metallurgical structure (graphite flake) that is difficult to machine. Consequently, a substantial problem exists in producing a roller with a cylindrical tungsten-carbide surface and a true bore to receive a journal. Additionally, in many applications, the material of the roller must have a high compressive yield strength to avoid cold-flow forming in use. Resistance to impact and shock are also important. Of course, in addition to the above characteristics, as suggested above, the matrix metal (alloy) must have: good fluidity to penetrate the particle layer, and low surface tension to wet the particles. These specifications in combination present a substantial problem. Accordingly, other aspects of the present invention reside in post-casting treatment of the roller and alloy material.